Synthesis gas (hereinafter referred to as syngas) is a mixture of hydrogen (H2) and carbon monoxide (CO). Syngas can be produced, in principle, from virtually any material containing carbon. Carbonaceous materials commonly include fossil resources such as natural gas, petroleum, coal, and lignite; and renewable resources such as lignocellulosic biomass and various carbon-rich waste materials. It is preferable to utilize a renewable resource to produce syngas because of the rising economic, environmental, and social costs associated with fossil resources.
There exist a variety of conversion technologies to turn these feedstocks into syngas. Conversion approaches can utilize a combination of one or more steps comprising gasification, pyrolysis, steam reforming, and/or partial oxidation of a carbon-containing feedstock.
Syngas is a platform intermediate in the chemical and biorefining industries and has a vast number of uses. Syngas can be converted into alkanes, olefins, oxygenates, and alcohols. These chemicals can be blended into, or used directly as, diesel fuel, gasoline, and other liquid fuels. Syngas can also be directly combusted to produce heat and power.
Since the 1920s it has been known that mixtures of methanol and other alcohols can be obtained by reacting syngas over certain catalysts (Forzatti et al., Cat. Rev.-Sci. and Eng. 33(1-2), 109-168, 1991). Fischer and Tropsch observed around the same time that hydrocarbon-synthesis catalysts produced linear alcohols as byproducts (Fischer and Tropsch, Brennst.-Chem. 7:97, 1926).
More recently, technology developers for these catalysts have included Dow Chemical/Union Carbide and Institut Francais du Petrole. Dow Chemical and Union Carbide jointly developed a sulfided mixed-alcohol catalyst based on molybdenum, MoS2 (Phillips et al., National Renewable Energy Laboratory TP-510-41168, April 2007). U.S. Pat. No. 4,752,623 (Stevens and Conway), originally assigned to Dow Chemical, discloses a cobalt-molybdenum-sulfide catalyst for producing mixed alcohols from syngas.
To produce these cobalt-molybdenum-sulfide catalysts, according to U.S. Pat. No. 4,752,623 and other related patents, ammonium heptamolybdate is dissolved in water and heated to 60° C., and then ammonium sulfide is added to make [NH4]2MoS4. In a separate container, cobalt acetate is dissolved. These two solutions are added simultaneously to a third vessel containing heated acetic acid. The catalyst precursor precipitates. The precipitate is filtered, dried in air, and calcined at 500° C. This chemistry can be difficult, with certain compounds undesirably precipitating out of solution.
In light of the shortcomings in the art, improved methods of making effective cobalt-molybdenum-sulfide catalysts are needed. These methods should identify important factors and conditions during the synthesis of preferred catalysts, so that a person of ordinary skill in the art can make and use these catalysts to commercially produce alcohols, such as ethanol, from syngas.